He's a hoarder! He WOULD buy more bags etc!
S - 'Ere Nathan, what you buyin more bin bags for?'
N - 'Cause these bags look stronger than them others love and I do have a lot a lot a lotta stuff to clear out'
S - 'Oh okay then' ...
How? Why would Shauna notice that he'd filled his old hoarded suitcases with a chopped up body? When your house is full of THAT much stuff, you don't notice things any more.
Also, the reason we're all here is that we are nosy, inquisitive, questioning, observant, half intelligent people ... not everyone thinks like we do! Just because YOU think YOU would notice all these little things and immediately realise your fella had bumped off his step sister, doesn't mean your average Joe/Jane would.
Shauna's dna on a bag knot isn't enough proof for me that she did anything - bags don't come clinically sterile wrapped.
If this bag was taken downstairs and put somewhere, she could've picked it up and moved it (if there's a knotted bin bag, I'd shove my fingers through the knot to pick it up) or maybe she pulled a bag from the roll for her own rubbish ...
My other ideas of deception DO happen while the 2 people are together!
It wasn't a "body shaped bag" was it!
It matters not one bit to me whether she ultimately is convicted or not, I'm not about to start having 'Free Shauna' T-shirts printed ... I'm just trying to assumed she's innocent UNTIL I hear the BOOM - there's your beyond reasonable doubt evidence.
ETA - Just because Nathan tells us he has Fibro doesn't mean he actually has fibro and even if at some point a Dr has said 'ooh I think you might have fibro' it doesn't necessarily mean that in the heat of the moment, when necessity calls, and adrenalin kicks in, that he couldn't do what needs to be done.